1. Field of the Invention
The present invention relates generally to nuclear reactor fuel assemblies, and more particularly, to improved dashpot constructions for a control rod guide thimble of the fuel assembly.
2. Description of the Prior Art
In a typical nuclear reactor, the reactor core includes a large number of fuel assemblies each composed of top and bottom nozzles with a plurality of elongated transversely spaced hollow guide thimbles extending longitudinally between the nozzles and a plurality of transverse support grids axially spaced along and attached to the guide thimbles. A plurality of elongated fuel elements or rods are supported by the transverse grids between top and bottom nozzles and transversely spaced apart from one another and from the guide thimbles. The fuel rods contain fissile material and are grouped by the grids in an array which provides a neutron flux in the core sufficient to support a high rate of nuclear fission and thus the release of a large amount of energy in the form of heat. A liquid coolant is pumped upwardly through the core in order to extract some of the heat generated in the core for the production of useful work.
Clusters of elongated control rods are mounted to drive mechanisms above the fuel assemblies and in alignment with the hollow guide thimbles. During operation of the nuclear reactor, the control rods can be inserted into the fuel assemblies via the channels defined therein by the hollow guide thimbles. The control rods are used to control the nuclear reaction occurring within the fuel assemblies of the reactor core, allowing power level to increase upon withdrawal and reducing power level upon insertion of the control rods.
In case of a need to quickly reduce power in the reactor core, the control rods are released from their drive mechanism and allowed to fall rapidly into the fuel assembly. If not controlled, the impact of the control rods on the fuel assembly could conceivably damage it. Therefore, deceleration and braking of the control rods is accomplished by providing a constriction to the flow of coolant in the form of a reduced diameter lower end portion of each guide thimble. Coolant flowing upwardly through the lower end portion of the guide thimble becomes trapped by the entering control rod forming a dashpot which effectively decelerates and brakes the control rod prior to impact: with the fuel assembly.
Historically, the dashpot has been formed by swaging the lower end portion of the guide thimble to a smaller diameter size, leaving the remainder of the guide thimble at the desired diameter size. This process is expensive to perform and forms a conical transition between the reduced diameter lower end portion and the remainder of the tube which is difficult to control.
The reduced diameter lower end portion of the prior art dashpot design also requires additional components to position and attach the lowermost or bottom grid or grids of the fuel assembly in place, further increasing the expense and difficulty of manufacture. The rest of the grids and a top nozzle fitting are assembled and joined by means for a bulged mechanical swaging of the guide thimble onto a plurality of identical larger sleeves that attach to the grids and to the top nozzle.
Consequently, a need exists for an alternative approach to dashpot construction so as to overcome the problems associated with the prior art dashpot design. One approach is disclosed in U.S. Pat. No. 4,655,990 to Leclercq wherein the constriction is formed by insertion of a smaller diameter tube or tube sections inside the lower end portion of the guide thimble. While the particular constructions of this patent provide a start in the right direction, they are not perceived as an optimum solution to all of the problems.